Method of manufacturing electric resistances



11 ill 'iiHlillU '1 W. MEYER ii .H

Filed Dec. 21, 1954 I' H H Oct. 8, 1935.

iatentecl Oct. 8, 1935 UNITED STATES PATENT OFFICE METHOD OF MANUFACTURING ELECTRIC RESISTANCES Germany Application December 21, 1934, Serial No. 758,670 In Germany August 2, 1933 7 Claims.

My invention relates to improvements in electric resistances and in the method of manufacturing the same, and more particularly in electric resistance of the type shown and described in the application for U. S. patent of Meyer and Berg, Ser. No. 682,946, filed July 29, 1933, for Electric resistance having negative temperature coefficients. As described in the said application for patent the resistance consists of a body of uranium dioxide which has a very high negative temperature coeificient, and which has a long life and may be used for many thousands of hours without practically varying the value of its resistance. As stated in the said application such resistances may be used in connection with systems for preventing high initial current impulses when a circuit is closed and for various other purposes, such for example as the regulation of the voltage of a system. In my opinion the fact that resistances of the type described in the application remain constant for a very long time may be explained by the oxygen content of the sintered body of uranium dioxide remaining absolutely constant. Such resistances may be manufactured so that at elevated temperature all the resistance bodies have, within slight limits, uniform resistances even if large numbers of the bodies are simultaneously manufactured. However, at ordinary temperature the resistances of the said bodies are subject to variations. The fact that notwithstanding this difference in the resistances at ordinary temperature the resistances at elevated temperatures are alike is based on the fact that a larger resistance is associated with a high temperature coeflicient, and a small resistance with a small temperature coeflicient. So far the said resistances have been used under such conditions only in which uniform resistances at elevated temperature were required, while it was sufficient that at ordinary temperature theresistance was not below a certain limit. However, for various purposes it is necessary to manufacture resistance bodies on a large scale the resistances of which are alike also at ordinary temperature. Therefore, the object of the improvements is to provide resistance bodies consisting of uranium dioxide the resistance of which is uniform at ordinary temperature, and with this object in view my invention consists in treating the resistance body consisting of uranium dioxide (U02) before or after sintering in an indifierent or reducing atmosphere and at a temperature which exceeds the temperature to which the re- I sistance body is to be subjected in practical service, and at least at a temperature of 200 C., the

said indifferent or reducing atmosphere containing a small amount of oxygen. During such treatment at elevated temperature the partial pressure of the oxygen should be kept as far as possible constant, and say at about one millime- 5 ter of mercury or less. In case of indifierent gases such as nitrogen and the like this may be accomplished by admixing with the gases a suitable amount of oxygen, and in case of reducing gases the desired slight partial pressure of oxygen 0 is obtained by admixing vaporized water with the reducing gases.

After such treatment resistance bodies which otherwise have very difierent resistance values have practically uniform resistance values at ordinary temperature. In my opinion this may be explained by the fact that the difierent resistance bodies absorb an exceedingly slight amount or traces of oxygen, the volumes absorbed by all the resistance bodies being alike. But I wish it to be understood that the stage of oxidation of the uranium dioxide is not in any way altered by the said absorption of oxygen. The absorbed traces of oxygen cannot be proved by the ordinary analytic methods, and the body consists as before of uranium dioxide. I believe that the oxygen is only occluded or dissolved in the said bodies.

During the said heating of the resistance body the change of the resistance caused by the absorption of oxygen takes place already at a temperature of 200 C. But since in many cases the resistance bodies are used at temperature intervals between ordinary temperature and from 400 to 600 C. it is preferred in the manufacture of the body to cause the absorption of the oxygen at a temperature of about 1000" C. and for this purpose to heat the bodies in the indiiferent or reducing atmosphere containing oxygen at a temperature of about 1000 0., so that within the 0 limits of the temperatures to which the resistance is subjected in use there is no variation of the oxygen content.

In order that the invention be more clearly understood a resistance body and an apparatus for treating the same have been shown in the accompanying drawing. In said drawing,

Fig. 1 is a sectional elevation showing the apparatus for treating the body, and

Fig. 2 is a sectional elevation showing a resistance body confined within a closed container.

Referring to Fig. l the apparatus shown by way of example comprises a pipe a made from refractory material such as chamotte and having a length of from 2 to 3 meters. For heating same. The upper tape of an endless band '0 trained on rollers d. is passed through the said pipe, the upper tape moving in the direction of the arrow 1:. At the left hand or delivery end a bottle e is located which is partly filled with sulfuric acid. Through the stopper a, pipe I is passed into the said bottle which extends nearly to the bottom thereof, and a second pipe is passed through the stopper which ends above the level of the sulfuric acid, and which is directed with its outer end into the pipe a.

By means of the resistance coil b the pipe is heated at a temperature of 1000 C. The resistance bodies to be treated are put into cups h which are placed on the upper tape of the band 0 at the right hand or intake end-thereof, the said bodies being either in a state before sintering or in a sintered state. The velocity of the tape is about 60 cm. per hour. While the bodies thus travel through the heated pipe a nitrogen containing about 0,1% oxygen is supplied to the ppie f, and after being cleaned within the sulfuric acid contained in the bottle c it is delivered through the pipe g in a direction opposite to the movement of the cups h. At the elevated temperature the resistance bodies are sintered, if they have not before been sintered, and they take up traces of oxygen suflicient to accomplish the result stated above.

If a reducing gas such as hydrogen is supplied to the apparatus the bottle e may be partly filled with water. The hydrogen flowing through the said bottle takes up a suflicient amount of vaporized water, the said water being dissociated at the elevated temperature within the pipe a for supplying the oxygen needed for acting on the resistance bodies.

4 In service the resistance body must be protected from the access of oxygen, in order that the desired oxygen content be not altered or increased. For this reason the resistance body is mounted within a closed container 7: filled with an indiiferent or reducing gas such as nitrogen,

9,017,01 1 the pipe a resistance coil b is wound around the argon, hydrogen or a mixture of such gases, or within a highly evacuated container, as is shown in Fig. 2.

I claim:

1. In the method of manutacturing resistance bodies consisting of sintered uranium dioxide, the step which consists in exposing a body of uranium dioxide to a temperature of at least 200 C. in a non-oxidizing atmosphere having a slight partial pressure of oxygen, the said partial pressure of 10 oxygen being less than is needed for changing the stage of oxidation of the uranium dioxide.

2. The method as claimed in claim 1, in which the atmosphere to which the resistance body is exposed is a reducing atmosphere containing va- 1:; porized water.

3. The method as claimed in claim 1, in which the atmosphere to which the resistance body is exposed is an indiflerent atmosphere containing oxygen.

4. The method of treating resistance bodies of sintered uranium dioxide which comprises heating said bodies in a non-oxidizing atmosphere having a slight partial pressure of oxygen to a temperature of not less than 200' C., thepartial pressure of oxygen being insumcient to change the stage of oxidation of the uranium dioxide.

5. The method of treating sintered uranium dioxide electrical resistances comprising heating a body of uranium dioxide to a temperature of about 1000 C. in a non-oxidizing atmosphere having a partial pressure of oxygen less than that required for changing the stage of oxidation of the uranium dioxide.

6. The method claimed in claim 1 in which the 5 partial pressure of oxygen in the atmosphere during the exposure oi the bodies to the temperature of at least 200 C. is maintained constant and at less than 1 millimeter of mercury.

7. The method claimed in claim 4 in which the slight partial pressure of oxygen is less than 1 millimeter of mercury.

WILFRIED MEYER. 

